Various examples of osmotic drug delivery systems, in the form of tablets, for example, are known. In one example, a semipermeable membrane surrounds a core portion which includes a pharmaceutically active agent. In such systems, the pharmaceutically active agent is released at a constant rate, i.e., a zero order release profile.
In another example, a semipermeable membrane surrounds a core portion which includes a pharmaceutically active agent. An example of such a system is disclosed in U.S. Pat. No. 4,801,461. A layer of the pharmaceutically active agent is on the exterior of the semipermeable membrane. In such systems, the pharmaceutically active agent which is exterior to the semipermeable membrane is released quickly, and in general provides a quick and initial relief of symptoms to a patient. This initial release of the pharmaceutically active agent is at a first constant rate, i.e., a first rapid zero order release profile, also known as an immediate release. After the release of the pharmaceutically active agent which is exterior to the semipermeable membrane, the pharmaceutically active agent in the core portion is released at a second constant ratio, i.e., a second zero order release profile. Although this system may provide an initial quick release of the pharmaceutically active agent, in some cases there may be an excess initial release of the pharmaceutically active agent, or “dose dumping,” which is undesirable. Also, in some cases, the pharmaceutically active agent which is on the exterior of the semipermeable membrane may be degraded prematurely by saliva, hydrochloric acid, or other digestive juices and gastric juices.